1. Field of the Invention
The present invention relates to the transfer molding of semiconductor devices. More specifically, the present invention relates to a method of using a dam in transfer molding encapsulation of a semiconductor device and a heat sink, and in the resulting semiconductor device assembly.
2. State of the Art
A semiconductor integrated circuit (IC) device (referred to as a die or chip) includes bond pads on the active surface thereof for interfacing the integrated circuits of the semiconductor device with other circuits outside the die located on differing substrates. Since the semiconductor devices are relatively small and the attendant bond pads on the active surface thereof are, in comparison, considerably smaller, lead frames having a plurality of leads thereon connected to the bond pads of a semiconductor device are used to connect the semiconductor device with other circuits on differing substrates.
In a conventional lead frame design for use with an integrated circuit semiconductor device, the lead frame includes a plurality of leads having their ends terminating adjacent a side or edge of the integrated circuit semiconductor device with the device being supported by the die paddle portion of the lead frame. Electrical connections are made by means of wire bonds extending between the leads of the lead frame and the bond pads located on the active surface of the integrated circuit semiconductor device.
Subsequent to the wire bonding operation, portions of the leads of the lead frame and the integrated circuit semiconductor device may be encapsulated in suitable plastic material to form a packaged semiconductor device assembly. The leads and lead frame are then trimmed and formed to the desired configuration after the packaging of the semiconductor device in the encapsulant material.
In a Leads-Over-Chip (LOC) type lead frame configuration for an integrated circuit semiconductor (IC) device assembly, the leads of the lead frame extend over the active surface of the semiconductor device being insulated therefrom by tape which is adhesively bonded to the semiconductor device and the leads of the lead frame. Electrical connections are made between the leads of the lead frame and bond pads on the active surface of the semiconductor device by way of wire bonds extending therebetween. After wire bonding, the leads of the LOC lead frame and the semiconductor device are encapsulated in suitable plastic to encapsulate the semiconductor device and portions of the leads. Subsequently, the leads are trimmed and formed to the desired configuration to complete the packaged semiconductor device.
By far the most common manner of forming a plastic package about a semiconductor device assembly is molding and, more specifically, transfer molding. In this process, with specific reference to an LOC type semiconductor die assembly, a semiconductor die is suspended by its active surface from the underside of inner lead extensions of a lead frame (typically Cu or Alloy 42) by a tape, screen print or spin-on dielectric adhesive layer. The bond pads of the die and the inner lead ends of the frame are then electrically connected by wire bonds (typically Au, although Al and other metal alloy wires have also been employed) by means known in the art. The resulting LOC die assembly, which may comprise the framework of a dual-in-line package (DIP), zig-zag in-line package (ZIP), small outline j-lead package (SOJ), quad flat pack (QFP), plastic leaded chip carrier (PLCC), surface mount device (SMD) or other plastic package configuration known in the art, is placed in a mold cavity and encapsulated in a thermosetting polymer which, when heated, reacts irreversibly to form a highly cross-linked matrix no longer capable of being re-melted.
The thermosetting polymer generally is comprised of three major components: an epoxy resin, a hardener (including accelerators), and a filler material. Other additives such as flame retardants, mold release agents and colorants are also employed in relatively small amounts. While many variations of the three major components are known in the art, the focus of the present invention resides in the filler materials employed and their effects on the active die surface.
Filler materials are usually a form of fused silica, although other materials such as calcium carbonates, calcium silicates, talc, mica and clays have been employed for less rigorous applications. Powdered, fused quartz is currently the primary filler used in encapsulants. Fillers provide a number of advantages in comparison to unfilled encapsulants. For example, fillers reinforce the polymer and thus provide additional package strength, enhance thermal conductivity of the package, provide enhanced resistance to thermal shock, and greatly reduce the cost of the polymer in comparison to its unfilled state. Fillers also beneficially reduce the coefficient of thermal expansion (CTE) of the composite material by about fifty percent in comparison to the unfilled polymer, resulting in a CTE much closer to that of the silicon or gallium arsenide die. Filler materials, however, also present some recognized disadvantages, including increasing the stiffness of the plastic package, as well as the moisture permeability of the package.
When a heat sink is used on a semiconductor device assembly package, encapsulation of the semiconductor device becomes more difficult during the transfer molding process. In the first instance, the inclusion of the heat sink along with the semiconductor device attached to the lead frame makes the transfer molding of the assembly more difficult as more components must be placed and aligned within the mold cavity. Misalignment of the semiconductor device and the heat sink within the mold cavity may cause bleeding and flashing of the resin molding compound over the heat sink. Furthermore, when the heat sink, which is usually copper or an alloy thereof, rests against the mold surface during the transfer molding process, damage to the mold surface can occur by the mold surface being scratched and/or worn from contact therewith by the heat sink. The resulting worn mold surfaces cause the resin molding compound to bleed and flash over the outside of the heat sink during the transfer molding process. This affects the ability of the heat sink to transfer heat to the surrounding environment during the operation of the semiconductor device as well as presenting a poor appearance of the molded semiconductor device assembly.
Accordingly, there is a need for an improved transfer molding process for packaging semiconductor devices having heat sinks associated therewith to help prevent or reduce the bleeding or flashing of the molding compound over portions of the heat sink during the transfer molding process of the semiconductor device assembly.